Expandable stents and method for making same

ABSTRACT

The invention is directed to an expandable stent for implantation in a body lumen, such as an artery, and a method for making it from a single length of tubing. The stent consists of a plurality of radially expandable cylindrical elements generally aligned on a common axis and interconnected by one or more interconnective elements. The individual radially expandable cylindrical elements consist of ribbon-like material disposed in an undulating pattern. Portions of the expanded stent project outwardly into engagement with the vessel wall to more securely attach the dtent.

RELATED APPLICATIONS

This application is a division of U.S. Ser. No. 08/556,516, filed Nov.13, 1995 now U.S. Pat. No. 5,603,721, which is a division of U.S. Ser.No. 08/281,790, filed Jul. 28, 1994, now U.S. Pat. No. 5,514,154, whichis a continuation-in-part of U.S. patent application Ser. No. 08/164,986filed Dec. 9, 1993, which is a continuation application of U.S. Ser. No.07/783,558 filed Oct. 28, 1991, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to expandable endoprosthesis devices, generallycalled stents, which are adapted to be implanted into a patient's bodylumen, such as blood vessel, to maintain the patency thereof. Thesedevices are very useful in the treatment of atherosclerotic stenosis inblood vessels.

Stents are generally tubular-shaped devices which function to hold opena segment of a blood vessel or other anatomical lumen. They areparticularly suitable for use to support and hold back a dissectedarterial lining which can occlude the fluid passageway therethrough.

Further details of prior art stents can be found in U.S. Pat. No.3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,338 (Balko et al.); U.S.Pat. No. 4,553,545 (Maass et al.); U.S. Pat. No. 4,733,665 (Palmaz);U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882(Gianturco); U.S. Pat. No. 4,856,516 (Hillstead); and U.S. Pat. No.4,886,062 (Wiktor), which are hereby incorporated herein in theirentirety by reference thereto.

Various means have been described to deliver and implant stents. Onemethod frequently described for delivering a stent to a desiredintraluminal location includes mounting the expandable stent on anexpandable member, such as a balloon, provided on the distal end of anintravascular catheter, advancing the catheter to the desired locationwithin the patient's body lumen, inflating the balloon on the catheterto expand the stent into a permanent expanded condition and thendeflating the balloon and removing the catheter. One of the difficultiesencountered using prior stents involved maintaining the radial rigidityneeded to hold open a body lumen while at the same time maintaining thelongitudinal flexibility of the stent to facilitate its delivery.

What has been needed and heretofore unavailable is a stent which has ahigh degree of flexibility so that it can be advanced through tortuouspassageways and can be readily expanded and yet have the mechanicalstrength to hold open the body lumen into which it expanded. The presentinvention satisfies this need.

SUMMARY OF THE INVENTION

The present invention is directed to an expandable stent which isrelatively flexible along its longitudinal axis to facilitate deliverythrough tortuous body lumens, but which is stiff and stable enoughradially in an expanded condition to maintain the patency of a bodylumen such as an artery when implanted therein.

The stent of the invention generally includes a plurality of radiallyexpandable cylindrical elements which are relatively independent intheir ability to expand and to flex relative to one another. Theindividual radially expandable cylindrical elements of the stent aredimensioned so as to be longitudinally shorter than their own diameters.Interconnecting elements or struts extending between adjacentcylindrical elements provide increased stability and a preferableposition to prevent warping of the stent upon the expansion thereof. Theresulting stent structure is a series of radially expandable cylindricalelements which are spaced longitudinally close enough so that smalldissections in the wall of a body lumen may be pressed back intoposition against the lumenal wall, but not so close as to compromise thelongitudinal flexibilities of the stent. The individual cylindricalelements may rotate slightly relative to adjacent cylindrical elementswithout significant deformation, cumulatively giving a stent which isflexible along its length and about its longitudinal axis but is stillvery stiff in the radial direction in order to resist collapse.

The stent embodying features of the invention can be readily deliveredto the desired lumenal location by mounting it on an expandable memberof a delivery catheter, for example a balloon, and passing thecatheter-stent assembly through the body lumen to the implantation site.A variety of means for securing the stent to the expandable member onthe catheter for delivery to the desired location are available. It ispresently preferred to compress the stent onto the balloon. Other meansto secure the stent to the balloon include providing ridges or collarson the inflatable member to restrain lateral movement, or usingbioresorbable temporary adhesives.

The presently preferred structure for the expandable cylindricalelements which form the stents of the present invention generallycircumferential undulating pattern, serpentine. The transversecross-section of the undulating component of the cylindrical element isrelatively small and preferably has an apect ratio of about two to oneto about 0.5 to one. A one to one apect ratio has been foundparticularly suitable. The open reticulated structure of the stentallows for the perfusion of blood over a large portion of the arterialwall which can improve the healing and repair of a damaged arteriallining.

The radial expansion of the expandable cylinder deforms the undulatingpattern thereof similar to changes in a waveform which result fromdecreasing the waveform's amplitude and the frequency. Preferably, theundulating patterns of the individual cylindrical structures are inphase with each other in order to prevent the contraction of the stentalong its length when it is expanded. The cylindrical structures of thestent are plastically deformed when expanded (except with NiTi alloys)so that the stent will remain in the expanded condition and thereforethey must be sufficiently rigid when expanded to prevent the collapsethereof in use. During expansion of the stent, portions of theundulating pattern will tip outwardly resulting in projecting members onthe outer surface of the expanded stent. These projecting members tipradially outwardly from the outer surface of the stent and embed in thevessel wall and help secure the expanded stent so that it does not moveonce it is implanted.

With superelastic NiTi alloys, the expansion occurs when the stress ofcompression is removed so as to allow the phase transformation fromaustenite back to martensite and as a result the expansion of the stent.

The elongated elements which interconnect adjacent cylindrical elementsshould have a transverse cross-section similar to the transversedimensions of the undulating components of the expandable cylindricalelements. The interconnecting elements may be formed in a unitarystructure with the expandable cylindrical elements from the sameintermediate product, such as a tubular element, or they may be formedindependently and connected by suitable means, such as by welding or bymechanically securing the ends of the interconnecting elements to theends of the expandable cylindrical elements. Preferably, all of theinterconnecting elements of a stent are joined at either the peaks orthe valleys of the undulating structure of the cylindrical elementswhich for the stent. In this manner there is no shortening of the stentupon expansion.

The number and location of elements interconnecting adjacent cylindricalelements can be varied in order to develop the desired longitudinalflexibility in the stent structure both in the unexpanded as well as theexpanded condition. These properties are important to minimizealteration of the natural physiology of the body lumen into which thestent is implanted and to maintain the compliance of the body lumenwhich is internally supported by the stent. Generally, the greater thelongitudinal flexibility of the stent, the easier and the more safely itcan be delivered to the implantation site.

In a presently preferred embodiment of the invention the stent isconveniently and easily formed by coating stainless steel tubing with amaterial resistant to chemical etching, removing portions of the coatingto expose portions of underlying tubing which are to be removed todevelop the desired stent structure. The exposed portions of the tubingare removed by chemically etching from the tubing exterior leaving thecoated portion of the tubing material in the desired pattern of thestent structure. The etching process develops smooth openings in thetubing wall without burrs or other artifacts which are characteristic ofmechanical or laser machining processes in the small sized productscontemplated. Moreover, a computer controlled laser patterning processto remove the chemical resistive coating makes photolithographytechnology adaptable to the manufacture of these small products. Theforming of a mask in the extremely small sizes needed to make the smallstents of the invention would be a most difficult task. A plurality ofstents can be formed from one length of tubing by repeating the stentpattern and providing small webs or tabs to interconnect the stents.After the etching process, the stents can be separated by severing thesmall webs or tabs which connect them.

Other features and advantages of the present invention will become moreapparent from the following detailed description of the invention. Whentaken in conjunction with the accompanying exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view, partially in section, of a stentembodying features of the invention which is mounted on a deliverycatheter and disposed within a damaged artery.

FIG. 2 is an elevational view, partially in section, similar to thatshown in FIG. 1 wherein the stent is expanded within a damaged artery,pressing the damaged lining against the arterial wall.

FIG. 3 is an elevational View, partially in section showing the expandedstent within the artery after withdrawal of the delivery catheter.

FIG. 4 is a perspective view of a stent embodying features of theinvention in an unexpanded state, with one end of the stent being shownin an exploded view illustrate the details thereof.

FIG. 5 is a plan view of a flattened section of a stent of the inventionwhich illustrates the undulating pattern of the stent shown in FIG. 4.

FIG. 6 is a schematic representation of equipment for selectivelyremoving coating applied to tubing in the manufacturing of the stents ofthe present invention.

FIGS. 7 through 10 are perspective views schematically illustratingvarious configurations of interconnective element placement between theradially expandable cylindrical elements of the stent.

FIG. 11 is a plan view of a flattened section of a stent illustrating analternate undulating pattern in the expandable cylindrical elements ofthe stent which are out of phase.

FIG. 12 is an enlarged partial view of the stent of FIG. 5 with thevarious members slightly expanded.

FIG. 13 is a perspective view of the stent of FIG. 4 after it is fullyexpanded depicting some members projecting radially outwardly.

FIG. 14 is an enlarged, partial perspective view of one U-shaped memberwith its tip projecting outwardly after expansion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a stent 10 incorporating features of the inventionwhich is mounted onto a delivery catheter 11. The stent generallycomprises a plurality of radially expandable cylindrical elements 12disposed generally coaxially and interconnected by elements 13 disposedbetween adjacent cylindrical elements. The delivery catheter 11 has anexpandable portion or balloon 14 for expanding of the stent 10 within anartery 15. The artery 15, as shown in FIG. 1 has a dissected lining 16which has occluded a portion of the arterial passageway.

The delivery catheter 11 onto which the stent 10 is mounted, isessentially the same as a conventional balloon dilatation catheter forangioplasty procedures. The balloon 14 may be formed of suitablematerials such as polyethylene, polyethylene terephthalate, polyvinylchloride, nylon and ionomers such as Surlyn® manufactured by the PolymerProducts Division of the Du Pont Company. Other polymers may also beused. In order for the stent 10 to remain in place on the balloon 14during delivery to the site of the damage within the artery 15, thestent 10 is compressed onto the balloon. A retractable protectivedelivery sleeve 20 as described in copending application Ser. No.07/647,464 filed on Apr. 25, 1990 and entitled STENT DELIVERY SYSTEM maybe provided to further ensure that the stent stays in place on theexpandable portion of the delivery catheter 11 and prevent abrasion ofthe body lumen by the open surface of the stent 20 during delivery tothe desired arterial location. Other means for securing the stent 10onto the balloon 14 may also be used, such as providing collars orridges on the ends of the working portion, i.e. the cylindrical portion,of the balloon.

Each radially expandable cylindrical element 12 of the stent 10 may beindependently expanded. Therefore, the balloon 14 may be provided withan inflated shape other than cylindrical, e.g. tapered, to facilitateimplantation of the stent 10 in a variety of body lumen shapes.

In a preferred embodiment, the delivery of the stent 10 is accomplishedin the following manner. The stent 10 is first mounted onto theinflatable balloon 14 on the distal extremity of the delivery catheter11. The balloon 14 is slightly inflated to secure the stent 10 onto theexterior of the balloon. The catheter-stent assembly is introducedwithin the patient's vasculature in a conventional Seldinger techniquethrough a guiding catheter (not shown). A guidewire 18 is disposedacross the damaged arterial section with the detached or dissectedlining 16 and then the catheter-stent assembly is advanced over aguidewire 18 within the artery 15 until the stent 10 is directly underthe detached lining 16. The balloon 14 of the catheter is expanded,expanding the stent 10 against the artery 15, which is illustrated inFIG. 2. While not shown in the drawing, the artery 15 is preferablyexpanded slightly by the expansion of the stent 10 to seat or otherwisefix the stent 10 to prevent movement. In some circumstances during thetreatment of stenotic portions of an artery, the artery may have to beexpanded considerably in order to facilitate passage of blood or otherfluid therethrough.

The stent 10 serves to hold open the artery 15 after the catheter 11 iswithdrawn, as illustrated by FIG. 3. Due to the formation of the stent10 from elongated tubular member, the undulating component of thecylindrical elements of the stent 10 is relatively flat in transversecross-section, so that when the stent is expanded, the cylindricalelements are pressed into the wall of the artery 15 and as a result donot interfere with the blood flow through the artery 15. The cylindricalelements 12 of stent 10 which are pressed into the wall of the artery 15will eventually be covered with endothelial cell growth which furtherminimizes blood flow interference. The undulating portion of thecylindrical sections 12 provide good tacking characteristics to preventstent movement within the artery. Furthermore, the closely spacedcylindrical elements 12 at regular intervals provide uniform support forthe wall of the artery 15, and consequently are well adapted to tack upand hold in place small flaps or dissections in the wall of the artery15 as illustrated in FIGS. 2 and 3.

FIG. 4 is an enlarged perspective view of the stent 10 shown in FIG. 1with one end of the stent shown in an exploded view to illustrate ingreater detail the placement of interconnecting elements 13 betweenadjacent radially expandable cylindrical elements 12. Each pair of theinterconnecting elements 13 on one side of a cylindrical element 12 arepreferably placed to achieve maximum flexibility for a stent. In theembodiment shown in FIG. 4 the stent 10 has three interconnectingelements 13 between adjacent radially expandable cylindrical elements 12which are 120 degrees apart. Each pair of interconnecting elements 13 onone side of a cylindrical element 12 are offset radially 60 degrees fromthe pair on the other side of the cylindrical element. The alternationof the interconnecting elements results in a stent which islongitudinally flexible in essentially all directions. Variousconfigurations for the placement of interconnecting elements arepossible, and several examples are illustrated schematically in FIGS.7-10. However, as previously mentioned, all of the interconnectingelements of an individual stent should be secured to either the peaks orvalleys of the undulating structural elements in order to preventshortening of the stent during the expansion thereof.

FIG. 10 illustrates a stent of the present invention wherein threeinterconnecting elements 12 are disposed between radially expandablecylindrical elements 11. The interconnecting elements 12 are distributedradially around the circumference of the stent at a 120-degree spacing.Disposing four or more interconnecting elements 13 between adjacentcylindrical elements 12 will generally give rise to the sameconsiderations discussed above for two and three interconnectingelements.

The properties of the stent 10 may also be varied by alteration of theundulating pattern of the cylindrical elements 13. FIG. 11 illustratesan alternative stent structure in which the cylindrical elements are inserpentine patterns but out of phase with adjacent cylindrical elements.The particular pattern and how many undulations per unit of lengtharound the circumference of the cylindrical element 13, or the amplitudeof the undulations, are chosen to fill particular mechanicalrequirements for the stent such as radial stiffness.

The number of undulations may also be varied to accommodate placement ofinterconnecting elements 13, e.g. at the peaks of the undulations oralong the sides of the undulations as shown in FIGS. 5 and 11.

In keeping with the invention, and with reference to FIGS. 4 and 12-14,cylindrical elements 12 are in the form of a serpentine pattern 30. Aspreviously mentioned, each cylindrical element 12 is connected byinterconnecting elements 13. Serpentine pattern 30 is made up of aplurality of U-shaped members 31, W-shaped members 32, and Y-shapedmembers 33, each having a different radius so that expansion forces aremore evenly distributed over the various members.

As depicted in FIGS. 13 and 14, after cylindrical elements 12 have beenradially expanded, outwardly projecting edges 34 are formed. That is,during radial expansion U-shaped members 31 will tip outwardly therebyforming outwardly projecting edges. These outwardly projecting edgesprovide for a roughened outer wall surface of stent 10 and assist inimplanting the stent in the vascular wall by embedding into the vascularwall. In other words, outwardly projecting edges embed into the vascularwall, for example artery 15, as depicted in FIG. 3. Depending upon thedimensions of stent 10 and the thickness of the various members makingup the serpentine pattern 30, any of the U-shaped members 31, W-shapedmembers 32, and Y-shaped members 33 can tip radially outwardly to form aprojecting edge 34. It is most likely and preferred that U-shapedmembers 31 tip outwardly since they do not join with any connectingmember 13 to prevent them from expanding outwardly.

The stent 10 of the present invention can be made in many ways. However,the preferred method of making the stent is to coat a thin-walledtubular member, such as stainless steel tubing, with a material which isresistive to chemical etchants, remove portions of the coating to exposeunderlying tubing which is to be removed but to leave coated portions ofthe tubing in the desired pattern for the stent so that subsequentetching will remove the exposed portions of the metallic tubing, butwill leave relatively untouched the portions of the metallic tubingwhich are to form the stent. The coated portion of the metallic tube isin the desired shape for the stent. An etching process avoids thenecessity of removing burrs or slag inherent in conventional or lasermachining process. It is preferred to remove the etchant-resistivematerial by means of a machine-controlled laser as illustratedschematically in FIG. 6.

A coating is applied to a length of tubing which, when cured, isresistive to chemical etchants. "Blue Photoresist" made by the ShipleyCompany in San Jose, Calif., is an example of suitable commerciallyavailable photolithographic coatings. The coating is preferably appliedby electrophoretic deposition.

To ensure that the surface finish is reasonably uniform, one of theelectrodes used for the electrochemical polishing is a doughnut-shapedelectrode which is placed about the central portion of the tubularmember.

The tubing may be made of suitable biocompatible material such asstainless steel, titanium, tantalum, superelastic NiTi alloys and evenhigh strength thermoplastic polymers. The stent diameter is very small,so the tubing from which it is made must necessarily also have a smalldiameter. Typically the stent has an outer diameter on the order ofabout 0.06 inch in the unexpanded condition, the same outer diameter ofthe tubing from which it is made, and can be expanded to an outerdiameter of 0.1 inch or more. The wall thickness of the tubing is about0.003 inch. In the instance when the stent was plastic, it would have tobe heated within the arterial site where the stent is expanded tofacilitate the expansion of the stent. Once expanded, it would then becooled to retain its expanded state. The stent may be convenientlyheated by heating the fluid within the balloon or the balloon directlyby a suitable system such as disclosed in a co-pending application Ser.No. 07/521,337, filed Jan. 26, 1990 entitled DILATATION CATHETERASSEMBLY WITH HEATED BALLOON which is incorporated herein in itsentirety by reference. The stent may also be made of materials such assuperelastic NiTi alloys such as described in co-pending applicationSer. No. 07/629,381, filed Dec. 18, 1990, entitled SUPERELASTIC GUIDINGMEMBER which is incorporated herein in its entirety by reference. Inthis case the stent would be formed full size but deformed (e.g.compressed) into a smaller diameter onto the balloon of the deliverycatheter to facilitate transfer to desired intraluminal site. The stressinduced by the deformation transforms the stent from a martensite phaseto an austenite phase and upon release of the force, when the stentreaches the desired intraluminal location, allows the stent to expanddue to the transformation back to the martensite phase.

Referring to FIG. 6, the coated tubing 21 is put in a rotatable colletfixture 22 of a machine controlled apparatus 23 for positioning thetubing 21 relative to a laser 24. According to machine-encodedinstructions, the tubing 21 is rotated and moved longitudinally relativeto the laser 24 which is also machine controlled. The laser selectivelyremoves the etchant-resistive coating on the tubing by ablation and apattern is formed such that the surface of the tube that is to beremoved by a subsequent chemical etching process is exposed. The surfaceof the tube is therefore left coated in the discrete pattern of thefinished stent.

A presently preferred system for removing the coating on the tubingincludes the use an 80-watt CO₂ laser, such as a Coherent Model 44, inpulse mode (0.3 mS pulse length); 48 mA key current and 48 W key powerwith 0.75 W average power, at 100 Hz; Anorad FR=20; 12.5 Torr; with noassist gas. Low pressure air is directed through the fine focus head toensure that no vapor contacts the lens. The assist gas jet assembly onthe laser unit may be removed to allow a closer proximity of the finefocus head and the collet fixture. Optimum focus is set at the surfaceof the tubing. Cured photo-resist coating readily absorbs the energy ofthe CO₂ wavelength, so that it can be readily removed by the laser. Acoated 4-inch length of 0.06 inch stainless steel tubing is preferredand four stents can be patterned on the length of tubing. Three tabs orwebs between stents provide good handling characteristics for the tubingafter the etching process.

The process of patterning the resistive coating on the stent isautomated except for loading and unloading the length of tubing.Referring again to FIG. 6 it may be done, for example, using aCNC-opposing collet fixture 22 for axial rotation of the length oftubing, in conjunction with a CNC X/Y table 25 to move the length oftubing axially relative to a machine-controlled laser as described. Theentire space between collets can be patterned using the CO₂ laser set-upof the foregoing example. The program for control of the apparatus isdependent on the particular configuration used and the pattern to beablated in the coating, but is otherwise conventional.

This process makes possible the application of present photolithographytechnology in manufacturing the stents. While there is presently nopractical way to mask and expose a tubular photo-resist coated part ofthe small size required for making intravascular stents, the foregoingsteps eliminate the need for conventional masking techniques.

After the coating is thus selectively ablated, the tubing is removedfrom the collet fixture 22. Next, wax such at ThermoCote N-4 is heatedto preferably just above its melting point, and inserted into the tubingunder vacuum or pressure. After the wax has solidified upon cooling, itis reheated below its melting point to allow softening, and a smallerdiameter stainless steel shaft is inserted into the softened wax toprovide support. The tubing is then etched chemically in a conventionalmanner. After cutting the tabs connecting the stents any surfaceroughness or debris from the tabs is removed. The stents are preferablyelectrochemically polished in an acidic aqueous solution such as asolution of ELECTRO-GLO #300, sold by the ELECTRO-GLO CO., Inc. inChicago, Ill., which is a mixture of sulfuric acid, carboxylic acids,phosphates, corrosion inhibitors and a biodegradable surface activeagent. The bath temperature is maintained at about 110-135 degrees F.and the current density is about 0.4 to about 1.5 amps per in.² Cathodeto anode area should be at least about two to one. The stents may befurther treated if desired, for example by applying a biocompatiblecoating.

While the invention has been illustrated and described herein in termsof its use as an intravascular stent, it will be apparent to thoseskilled in the art that the stent can be used in other instances such asto expand prostatic urethras in cases of prostate hyperplasia. Othermodifications and improvements may be made without departing from thescope of the invention.

Other modifications and improvements can be made to the inventionwithout departing from the scope thereof.

What is claimed is:
 1. An intravascular stent for expanding and implanting in a body lumen, comprising:a plurality of spaced apart cylindrical rings positioned along a longitudinal axis, each of the cylindrical rings having a plurality of undulating elements in the form of a repeating pattern of substantially U-shaped members; and a plurality of connecting members for connecting adjacent cylindrical rings; and the cylindrical rings being positioned relative to each other so that the substantially U-shaped members of adjacent cylindrical rings are out of phase; the cylindrical rings having a delivery diameter and an implanted diameter so that as the cylindrical rings are expanded from the delivery diameter to the implanted diameter at lest some of the substantially U-shaped members project radially outwardly.
 2. The intravascular stent of claim 1, wherein at least some of the connecting members are substantially parallel to each other.
 3. The intravascular stent of claim 1, wherein the cylindrical rings and connecting members are formed from a single piece of hollow tubing.
 4. The intravascular stent of claim 1, wherein the substantially U-shaped members have a curved portion having a substantially uniform radius of curvature.
 5. The intravascular stent of claim 4, wherein at least some of the curved portions of the substantially U-shaped members deform when the stent is expanded from the delivery diameter, to the larger implanted diameter, the deformed curved portions projecting radially outwardly as the stent is expanded to the larger implanted diameter.
 6. The intravascular stent of claim 5, wherein the stent has a first end and a second end, at least some of the curved portions of the substantially U-shaped members forming the first end and the second end project radially outwardly when the stent is expanded from the delivery diameter to the larger implanted diameter.
 7. The intravascular stent of claim 1, wherein the cylindrical elements and the connecting members are formed from a flat sheet of material.
 8. The intravascular stent of claim 1, wherein the stent is formed from a metal alloy.
 9. The intravascular stent of claim 8, wherein the metal alloy is taken from the group of metal alloys comprising stainless steel and nickel-titanium.
 10. The intravascular stent of claim 1, wherein the connecting members between adjacent cylindrical elements are substantially the same length.
 11. The intravascular stent of claim 1, wherein the plurality of U-shaped members have substantially the same size and shape.
 12. The intravascular stent of claim 1, wherein at least five cylindrical elements are interconnected.
 13. An intravascular stent for expanding and implanting in a body lumen, comprising:a plurality of spaced-apart cylindrical rings positioned along a longitudinal axis, each of the cylindrical rings having a plurality of undulating elements in the form of a repeating pattern of substantially U-shaped members; each of the substantially U-shaped members having a pair of sides connected by a curved portion; a plurality of struts for connecting adjacent cylindrical rings; and the cylindrical rings being positioned relative to each other so that the substantially U-shaped members of adjacent cylindrical rings are out of phase; the cylindrical rings having a delivery diameter and an implanted diameter so that as the cylindrical rings are expanded from the delivery diameter to the implanted diameter at lest some of the substantially U-shaped members project radially outwardly. 